Generally, the performance of a radioelectric circuit forming at least one RF radiofrequency chain of analogue functions placed in series according to various combinations, such as for example an RF amplification, a frequency transposition, a filtering, are affected by unforeseeable and variable discrepancies resulting for example from the manufacture, from the sensitivity to the temperature of the environment, from the ageing of the components and their exposure to ionizing radiations.
In a particular manner, the performance of the circuits of satellite-borne telecommunications payloads undergo unforeseeable effects such as these.
Thus, for a conventional telecommunications payload comprising a small number of RF radiofrequency chains and antennas with a single source per beam SFPB (“Single Feed Per Beam”), a requirement is typically stipulated to estimate and to compensate the spectral response of the chains, so as to equalize the gain and the group propagation time on the useful band.
Dealing as one is with a telecommunications payload implementing an active antenna and a large number of associated RF radiofrequency chains or pathways, the requirement is stipulated not only of equalization for each pathway of the spectral response on the useful band, but also of equalization of the dispersions in phase and amplitude between the pathways on the useful band. Indeed, the radioelectric performance of active antennas is particularly sensitive to the phase shifts between pathways, and the requirement fixed to the tolerance of such phase shifts is particularly severe, in particular for embedding an anti-jamming function.
In order to control the dispersions in amplitude and phase per pathway or between pathways, a first family of conventional so-called static solutions has been implemented and is still used nowadays, in particular in the space sector.
A first static solution of the said first family consists in over-constraining the technical performance specification requirements of each component or item of equipment of the radiofrequency chain, so that the sum of the dispersions in gain and phase on the useful band that are caused by the whole set of components of the radiofrequency chain, remains compatible with the desired performance level, for the lifetime envisaged.
This first conventional technique which over-constrains the design, the manufacture, the provisioning, and the adjustment in the phase of integration of the components of the chain or chains, corresponds to a conservative approach. The effect of this approach is to significantly increase the manufacturing costs. Moreover, the residual dispersions not being compensated dynamically, the dispersions remain non-negligible and certain functions such as anti-jamming cannot always be carried out.
A second static solution of the said first family consists in characterizing the behaviour of the elements or equipment, in terms of sensitivity to temperature and/or to supply voltage, and then in embedding a static compensation function, using the measurement of temperature and/or supply voltage, and adjusted in the AIT (Assembly, Integration and Test) phase, specifically for each instance. This type of solution allows static correction but does not allow a capacity for adaptation to the real discrepancies which may worsen, in particular on account of ageing and of the effect of the radiations experienced by the components of the radiofrequency chain in orbit.
In order to remedy the drawbacks exhibited by the first and second static solutions of the first family of solutions, a second family of so-called dynamic solutions is described in the article by A. Lecointre et al., entitled “On-Board Self Calibration Techniques” and published at the ESA Workshop of 17-19 Apr. 2012: “ESA workshop on advanced flexible telecom payloads”. This document reviews the dynamic calibration techniques used hitherto or under development, and evaluates the effect of the calibration signal on the nominal service or the communication signal as a function of the calibration technique used.
Each dynamic solution, described in the article by A. Lecointre et al., consists in estimating the discrepancies in amplitude and/or in phase on the radiofrequency chain or chains, by using a known calibration signal, injected at input and extracted at output of the radiofrequency chain. The deformation of the calibration signal at the output of the radiofrequency chain makes it possible to estimate the real-time spectral response, and to compensate the defects of the response with a feedback loop. This type of dynamic, looped solution makes it possible to adapt advantageously to the defects, independently of their origin and of a non-predictable variability of their occurrence, such as a defect caused by ageing and/or radiations.
In order to be able to inject and extract the measurement signal serving for calibration, a first dynamic solution of the said second family consists in suspending the useful telecommunication service for a period of time with the drawback of degrading the quality of the service.
A second dynamic solution of the said second family consists in spreading the spectrum of the calibration signal so as to be able to superimpose it on the useful traffic signal without overly disturbing the said useful signal, and thus avoid interruption of the telecommunication service. However, in this case the calibration measurement is affected by a low ratio of the level of the calibration signal measured to the noise and interference level SNIRcal (“calibration Signal to Noise and Interference Ratio”) and the spreading of the measured point in terms of frequency, the estimation of the defects corresponding to an average of the defects over the spread band.
A third dynamic solution of the said second family consists in injecting the calibration signal on frequencies that are not used by the communication services on guard bands with the drawback of poor granularity of the frequencies used for estimating the dispersion in amplitudes and/or in phases on the band of the chain to be calibrated.
A first technical problem is to improve the quality of correction or compensation of the response of a radiofrequency chain on the useful band of the radiofrequency chain, which is obtained by a correction or compensation carried out by a method and a system for dynamic calibration using the techniques described hereinabove.
A second technical problem is to limit the interference level experienced by the calibration signal and the source of which is the useful communication signal when the compensation method is implemented without interruption of the useful communication service.
A third technical problem is to minimize the distortion caused by the calibration signal on the useful communication signal at the output of the chain under calibration when calibration is active.